Electrically driven screw-drivers (using an electric motor as a power source) may mostly be classified into two types, namely one which is directly connected to a commercial current source (100 V) and the other in which a current is supplied to a motor after reduction of its voltage with a transformer. In either case, a connection cord is indispensable for connecting the motor to the electric power source. These screw-drivers may utilize a higher electric output but may be operated portably in only the range of an extensible cord length. Thus, these types of electrically driven screw-drivers may be essentially used for an assembling process in a factory.
There is also known another type of electrically driven screw-driver which is of a charging type including a battery-replacement type. This type of screw-driver has a disadvantage of a lower output but has an advantage of portability due to the absence of a connecting cord. However, the conventional charging type of the screw-driver requires a possibly highest output of a motor and therefore a charging unit of a large capacity, resulting in a big and inconvenient appliance. Thus, it may well be stated that there has been no screw-driver of a charging type, which are small enough to be portable in a pocket, convenient in handling and resistant to a severe operational condition.
As mentioned hereinabove, a number of batteries are necessary as a power source for obtaining a higher and practical output for fastening, which prevents the screw-driver from being compact and light. Even a battery of a limited capacity may be used for obtaining a higher output if the large number of reduction steps are utilized in a reduction system connected to the motor. However, such higher reduction ratio brings about a decrease in rotation number of an output shaft, leading to an impractical appliance. Thus, the portable screw-driver must be sized and designed for balancing a necessary operational capacity and a sufficient electric power to provide a corresponding output (or a battery size).
In general, the operational capacity must be so large that the appliance becomes inevitably too big to be carried by one hand. Thus, there has been need for a very compact and light electrically driven screw-driver, which is enough resistant to a severe operational condition.
In view of the foregoing, the inventor has studied for eliminating the disadvantages of the conventional charging type of portable screw-driver in the prior art and for developing a very compact, light but convenient electrically driven screw-driver, and has now found out after strict analysis of an operational procedure of the electrically driven screw-driver (hereinafter referred to merely a "screw-driver" for simplification) that a screw may be fastened into an object, which has already been threaded for receiving the screw, almost without load for about 90% of the fastening procedure and that only a finishing step of about 10% requires an instantaneously strong fastening force. On the contrary, a so-called tapping screw, which is screwed into an object without a threaded hole, may be gradually moved into the object against a considerable resistance while simultaneously making a thread. In this case, most of the working amount is directed to the tapping step and a relatively small working amount is directed to a finishing step requiring a strong fastening force.
Since the single fastening procedure has two steps as described hereinabove, it has been found out that about 90% of the total working amount required in the prefastening or screwing step is advantageously performed mechanically, while the remaining 10% in the finishing step of the strong fastening may well be performed manually without a machanical force. Actually, an extent of output to be required in the tapping work as described hereinabove may be sufficiently supplied from a charging battery of a relatively small capacity by combination of a small motor and a suitable reduction system. However, the finishing step requires much more fastening force, resulting in a big appliance with a higher power source if the fastening force is supplied electrically and mechanically. The conventional portable screw-driver has always utilized a mechanical and electrical force for performing the whole procedure or two steps of operation as described hereinbefore, thus never producing a very compact and light screw-driver which is portable in a pocket.
It will be appreciated from the foregoing that if only the prefastening or screwing step occupying most of the total work is performed mechanically but the finishing step requiring a strong fastening force (which is consumed in a very short time) is carried out manually, the mechanical size and the battery capacity may be correspondingly reduced, thereby producing a truly compact and light portable screw-driver. It has been found out, however, that there are a number of problems to be solved in order to embody the idea as described hereinabove. Firstly, when only the screwing step is carried out mechanically with a motor but the finishing step of fastening is continued by a turning movement with a hand gripping the screw-driver, the hand is suffered from a strong reverse resistance for preventing the advancing movement of the screw, so that the reduction gear (and the motor) directly connected to the driver bit starts its reverse rotation, thus never achieving the fastening operation. It has also been found out, therefore, that any means for avoiding the reverse rotation is necessary to be inserted between the driver bit and the output shaft of the reduction system. In order to avoid only the reverse movement of the driver bit, according to a principle of rheostatic braking a positive pole of a DC motor as a power source may be short-circuited to a negative pole instantaneously upon discontinuation of the rotation.
Secondly, there is a problem how to terminate the prefastening or screwing step on an optimum timing. If the termination of the screwing step is carried out by a manual ON-OFF switching operation based on an operator's intuition, any skillful operator can not always terminate the rotation on a constant timing. Further, the requirement of such the operator's intuition fails to produce an automatic appliance. The maximum output obtained on the driver bit may be adjusted to a degree of an opposite resistance generated at the end of the prefastening or screwing step for mechanically discontinuating the rotation of the driver bit. However, this operation generates an excessive mechanical load on the whole appliance upon each repeated operation and thus may not be employed in the screw-driver for use in a severe operational condition. Therefore, it is absolutely necessary for the driver bit to be automatically discontinued on the spot for its rotation upon reaching the predetermined fastening torque (or the predetermined opposite resistance), in order to terminate the screwing step on the optimum timing. Since the opposite resistance may be varied depending on materials to be fastened such as steel, wood or plastics, it will be appreciated that the fastening torque is difficult to be controlled on the optimum value for automatic discontinuation.
In view of the foregoing, it has now been found out that the first problem may be solved by providing a ratchet mechanism (capable of transmitting a rotation in only the unidirection but preventing the same in the other direction) at a portion of a chuck having a removable driver bit. Further, it has been confirmed that the ratchet mechanism is conveniently provided with a switching means for preventing the transmission of rotation power in the opposite direction upon either of the forward or reverse rotation of the screw-driver.
It has also been found out that the second problem may be solved by providing a torque controlling system having a clutch mechanism at a so-called planetary reduction system which comprises planetary gears operatively connected to a pinion gear, which in turn is connected to an output shaft of an electric motor, and an internal gear meshing with the planetary gears.